Allow pyright to "see" core properly

Without this change, the following file raises errors (in strict mode)
and reveals the type as "Unknown". After this change, the type is
correctly revealed as "ImagingCore".

  # pyright: strict

  from PIL import Image

  img = Image.new('RGB', (10, 10))
  data = img.getdata()
  reveal_type(data)

src/PIL/Image.py

@@ -86,6 +86,9 @@ WARN_POSSIBLE_FORMATS: bool = False
 MAX_IMAGE_PIXELS: int | None = int(1024 * 1024 * 1024 // 4 // 3)
 
 
+if TYPE_CHECKING:
+    from . import _imaging as core
+else:
     try:
         # If the _imaging C module is not present, Pillow will not load.
         # Note that other modules should not refer to _imaging directly;